Process for the polymerization and copolymerization of alpha-olefins in fluidized bed

ABSTRACT

The present invention relates to a process for polymerizing or copolymerizing alpha-olefins comprising, on the one hand, a prepolymerization stage by contacting one or more alpha-olefins with a Ziegler catalyst system consisting of a solid catalyst containing essentially atoms of halogen, magnesium and transition metal, and a co-catalyst consisting of an organomatellic compound, and, on the other hand, a polymerization or copolymerization stage in a gas phase process by means of a fluidized bed, by contacting one or more alpha-olefins with the prepolymer produced in the first stage, process characterized in that the alpha-olefin prepolymer and polymer or copolymer are in the form of a powder consisting of particles having specific values for the transition metal content and the mean diameter by mass, and characterized in that the particle size distribution of the prepolymer is narrow and easily controlled.

This is a continuation of application Ser. No. 504,982, filed June 16,1983 now abandoned.

The present invention relates to a process for polymerizing orcopolymerizing alpha-olefins in the gas phase by means of a fluidisedbed.

It is known that catalyst systems for polymerizing and copolymerizingalpha-olefins, known as Ziegler-Natta catalysts, are obtained bycombining on the one hand a catalyst which is a compound of a transitionmetal belonging to Group IV, V or VI of the Periodic Table of Elementsand on the other hand a co-catalyst which is an organo-metallic compoundof a metal of Groups I to III of the said Table. The compounds mostfrequently used are on the one hand halogenated derivatives of titaniumand on the other alkylaluminium compounds or alkylaluminium chlorides.

It is known that one can polymerise alpha-olefins in the gas phase, forexample in a fluidised-bed reactor in which the solid polymer beingformed is maintained in the fluidised state by means of a rising gasstream containing the alpha-olefins to be polymerised. The gas mixtureleaving the reactor is generally cooled prior to being recycled to thereactor, with an additional quantity of alpha-olefins added. Thepolymerization may be carried out by means of a catalyst system of theZiegler-Natta type, introduced continuously or semi-continuously intothe fluidised-bed reactor. Withdrawal of the polymer produced may alsobe carried out continuously or semi-continuously.

The two compounds of the catalyst system--the catalyst and theco-catalyst--may be brought into contact with each other either beforebeing introduced into the fluidised-bed reactor or actually inside thereactor. Experience has shown, however, that in this case and whateverthe method employed the polymerization reaction starts up very abruptlyas soon as the catalyst or catalytic system is introduced into thefluidised-bed reactor, creating on the one hand localised speeding up ofthe reaction in the fluidised bed and on the other the bursting of solidparticles of catalyst and loss of control of the particle size. Theselocalised bursts of activity in the reaction produce overheating andgenerally lead to the formation of agglomerates and coagulation of thepolymer making up the fluidised bed.

It is also known that one can introduce the catalyst into thefluidised-bed reactor in association with an inorganic granular support.This inorganic granular support generally consists of refractory oxidessuch as alumina, silica, aluminium silicate or magnesium oxide. Thecatalyst is usually deposited or impregnated or precipitated on thisgranular support, which confers specific properties on the said catalystwhich are of interest for the technique of fluidised-bed polymerization,which properties relate in particular to the particle size, resistanceto abrasion and activity of the catalyst. Generally speaking it is alsoknown that the dispersion of a catalyst on a granular support of thistype is accompanied by an increase in the catalytic activity, especiallywhen the polymerization reaction commences. The problem associated withthis excess activity may be partially solved by dispersing the catalystover a sufficiently large quantity of support. Unfortunately thepolymers or copolymers obtained in this way have comparatively highcontents of inorganic residues and this has an adverse effect on theirproperties.

It is further known that one can use very active catalytic systemscomprising catalysts based on magnesium and transition metals, it beingpossible to obtain these catalysts by reacting organo-magnesiumcompounds with compounds of transition metals or by pulverisingmagnesium compounds with compounds of transition metals. In view oftheir particle size and their very high activity, these catalysts aresometimes introduced into the fluidised bed polymerization reactor inthe form of a prepolymer. In order to avoid overheating, it is desirableto employ relatively high fluidisation speeds, for example, betweenabout 5 and 10 times the minimum fluidisation speed, that is to say,generally between about 40 and 80 cm/sec. Now the prepolymers employedup till now have a mean diameter by mass which is too small and/or aparticle size distribution which is too broad to be able tosatisfactorily use fluidisation speeds as great as those recommended, ifone is to avoid an intensive entrainment of the particles outside thefluidised bed. In fact the use of such prepolymers necessitates thepresence of a zone known as a particle release zone, located above thefluidised bed and having a very large volume, together with devices tocheck and recycle the fine particles entrained outside the fluidisedbed, such as cyclones and filters.

In order to avoid this entrainment, it has also been proposed that oneshould limit the speed of fluidisation to comparatively low figures, forexample less than 3 times the minimum speed of fluidisation, that is tosay less than about 25 cm/sec. However, in order to eliminate the heatof polymerization properly, it has been found necessary in the prior arttechniques to attach a mechanical agitation system and/or a device forthe introduction, recovery and recycling of one or more readily volatileliquids.

It has now been found that it is possible to avoid the above drawbacksand to carry out polymerization or copolymerization of alpha-olefins inthe gas phase continuously, by means of a fluidised bed, in simplifiedapparatus making it possible to obtain polymers or copolymers ofalpha-olefins having a variety of properties and having very lowcontents of inorganic residues.

The object of the invention is therefore an improved process for theproduction of polyolefins which comprises in a first stage thecontacting of one or more alpha-olefins with a catalytic system of theZiegler-Natta type comprising on the other hand a solid catalystoccurring in the form of solid particles and containing essentiallyatoms of halogen, magnesium and transition metals belonging to GroupsIV, V and VI of the Periodic Table of Elements and, on the other hand, aco-catalyst consisting of organo-metallic compounds of metals of GroupsI to III of the said Table, in order to obtain a prepolymer in the formof solid particles, then in a second stage the contacting of the saidprepolymer with one or more alpha-olefins under polymerization orcopolymerization conditions in the gas phase by means of a fluidisedbed, to produce a polymer or copolymer of alpha-olefins directly in theform of a powder, the process being characterised in that:

(a) the prepolymer contains, per gramme, between 2×10⁻³ and 10⁻¹gramme-milliatom of transition metal and occurs in the form of particleshaving a mean diameter by mass comprised between 80 and 300 microns anda particle size distribution such that the ratio of the mean diameter bymass Dm to the mean diameter by number Dn is less than or equal to 3;

(b) the alpha-olefin polymer or copolymer contains, per gramme, lessthan 5×10⁻⁴ gramme-milliatom of transition metal and occurs in the formof a powder consisting of particles having a mean diameter by masscomprised between 300 and 1500, preferably between 400 and 1500 microns.

The catalysts employed according to the invention may be obtained byvarious processes, especially by those in which a magnesium compound isprecipitated at the same time as one or more compounds of transitionmetals. In particular they may be obtained by reacting anorgano-magnesium compound of the reactive Grignard type and a transitionmetal compound employed in its maximum valency form. Another techniquewhich is equally well known consists in reacting magnesium metal with analkyl halide and a transition metal compound in its maximum valencystate. Precipitation is generally accompanied by the reduction of thetransition metal compound to a compound wherein the transition metal isin one of its lower valency states. In view of the particle sizeproperties of these catalysts, it is necessary to proceed to agranulometric separation operation such as screening of the catalystand/or the prepolymer itself in order to get the desired particle sizedistribution, it being known that the particle size distribution of theprepolymer is similar to that of the catalyst from which it originates.

One may also use a catalyst consisting of a transition metal compounddeposited on a magnesium compound support such as magnesium chloride,magnesium hydroxychloride, magnesium alcoholates or magnesium acetate.In particular one can use a support of the kind obtained by pulverisinganhydrous magnesium chloride or by the precipitation reaction of anorgano-magnesium compound and a halogenated compound. In this case, too,a granulometric selection operation, for example screening of thesupport, the catalyst and/or the prepolymer usually has to be performed.

According to one preferred mode of embodiment of the invention it ispossible to avoid the above-mentioned granulometric selection operationby using, as catalyst support, particles of magnesium chloride obtainedby reacting an organo-magnesium compound and a chlorinated organiccompound, complying with the following conditions:

the organo-magnesium compound is either a magnesium dialkyl of theformula R₁ MgR₂, or an organo-magnesium derivative of the formula R₁MgR₂.xAl(R₃)3, in which formulae R₁, R₂ and R₃ are identical ordifferent alkyl radicals having 2 to 12 carbon atoms and x is comprisedbetween 0.001 and 10, and preferably between 0.01 and 2.

the chlorinated organic compound is an alkyl chloride of the formula R₄Cl, in which R₄ is a secondary or preferably a tertiary alkyl radicalhaving 3 to 12 carbon atoms.

the reaction is carried out in the presence of an electron donorcompound which is an organic compound comprising at least one atom ofoxygen, sulphur, nitrogen and/or phosphorus. It may be chosen from amonga wide variety of products such as amines, amides, phosphines,sulphoxides, sulphones or ethers. Amongst electron donor compounds onemay select in particular an aliphatic ether oxide of the formula R₅ OR₆,in which R₅ and R₆ are identical or different alkyl radicals having 1 to12 carbon atoms. Moreover, the various reactants involved in thepreparation of such a support may be employed under the followingconditions:

the molar ratio of R₄ Cl:R₁ MgR₂ is comprised between 1.5 and 2.5 andpreferably comprised between 1.85 and 2.2; the molar ratio of R₄ Cl:R₁MgR₂.xAl (R₃)₃ is comprised between 1.5(1 +3/2x) and 2.5(1 +3/2x) andpreferably between 1.85(1 +3/2x) and 2.2(1 +3/2x);

the molar ratio between the electron donor compound and theorgano-magnesium compound (R₁ MgR₂ or R₁ MgR₂.xAl (R₃)₃ is comprisedbetween 0.01 and 2 and preferably comprised between 0.01 and 1;

the reaction between the organo-magnesium compound and the chlorinatedorganic compound takes place with agitation in a liquid hydrocarbon at atemperature comprised between 5 and 80° C.

When one operates in accordance with these conditions it is possible toobtain a magnesium chloride support having a mean diameter by masscomprised in particular between 10 and 100 microns and a particle sizedistribution such that Dm:Dn is less than or equal to 3. The catalystsand then the prepolymers prepared from these supports have a similarparticle size distribution, so that no granulometric selection operationsuch as screening is necessary.

The transition metal compounds may be deposited on the support accordingto methods which are well-known in themselves.

However, in order to obtain a catalyst with a better activity,especially for the polymerization of ethylene or the copolymerization ofethylene and higher alpha-olefins, and to obtain a prepolymer complyingwith the characteristics described according to the invention, it isadvantageous to deposit on the support by precipitation a transitionmetal compound such as a titanium compound whose valency is less than 4.This precipitation may be carried out according to known processes butis advantageously performed according to the following process:

the reaction of reducing a titanium compound of maximum valency, of theformula Ti(OR₇)₄ -nX_(n), in which R₇ is an alkyl group containing 2 to6 carbon atoms, X is a chlorine or bromine atom and n is an integer orfraction from 1 to 4 inclusive, is carried out by means of a reducingagent chosen from among the organo-magnesium compounds having theformula R₈ MgR₉ , in which R₈ and R₉ are identical or different alkylgroups containing from 2 to 12 carbon atoms, organo-zinc compounds ofthe formula Zn(R₁₀)₂ -yX_(y), in which R₁₀ is an alkyl radical with 2 to12 carbons, X is chlorine or bromine and y is 0 or 1 or a fraction lessthan 1, and organo-aluminium compounds of the formula Al (R₁₁)_(3-x)X_(x) in which R₁₁ is an alkyl group having 2 to 12 carbon atoms, X ischlorine or bromine and x is 0 or an integer or fraction not greaterthan 2;

the said reduction reaction is performed in the presence or absence ofan electron donor compound chosen from among organic compoundscomprising at least one atom of oxygen, sulphur, nitrogen and/orphosphorus, such as amines, amides, phosphines, sulphoxides, sulphonesor ethers; more particularly from among the ethers preference may begiven to aliphatic ether oxides of the formula R₁₂ OR₁₃, in which R₁₂and R₁₃ are identical or different alkyl groups with 1 to 12 carbonatoms;

the relative quantities of the various compounds (support, titaniumcompound, organo-magnesium, organo-zinc or organo-aluminium compounds,electron donor) by molar ratio are such that:

support:titanium compound is comprised between 1 and 50 and preferablycomprised between 2.5 and 10;

organo-magnesium or organo-zinc or organo-aluminium compound:titaniumcompound is less than 3 and preferably comprised between 0.5 and 1.5;

electron donor compound:titanium compound is comprised between 0 and 5and preferably comprised between 0.1 and 1.5.

The precipitation is performed at a temperature comprised between -30°and 100° C., with agitation, in a liquid hydrocarbon medium.

The way the reactants are used can vary. For example the reducing agent(organo-magnesium, organo-zinc or organo-aluminium compound) may beintroduced gradually into the liquid hydrocarbon medium containing thesupport and the titanium compound. It is also possible to introduce bothgradually and simultaneously the reducing agent and the titaniumcompound into the liquid hydrocarbon medium containing the support.However, it has been found that the best results are obtained when thesupport is impregnated by means of the reducing agent in a first stage,the reaction of the support thus impregnated with the tetravalenttitanium compound being carried out in a second stage, possibly in thepresence of an electron donor compound such as an aliphatic ether oxide.

In other cases, for example in the case of the polymerization ofpropylene or copolymerization of propylene with ethylene or otheralpha-olefins, catalysts are sought which not only have a satisfactoryactivity but also a very high degree of stereo-specificity, so as toobtain mainly isotactic polymers. Particularly good results are thenobtained by impregnating the magnesium chloride support with titaniumtetrachloride, this impregnation preferably being performed in thepresence of an electron donor compound. The preparation of thesecatalysts from supports is advantageously carried out in two stages,namely:

(a) a treatment of the support by means of an electron donor compound ofthe aromatic acid ester or aromatic ether type,

(b) an impregnation of the support thus treated by means of titaniumtetrachloride.

In the first stage the quantity of electron donor to be used iscomprised between 0.06 and 0.2 mole of electron donor per mole ofmagnesium compound of the support, and the temperature to be used iscomprised between about 20° and 50° C.

In the second stage the support is impregnated with pure titaniumtetrachloride or using titanium tetrachloride in a hydrocarbon medium;the quantities of titanium tetrachloride must be sufficient to fix onthe support from 0.5 to 3 atoms of titanium per 100 atoms of magnesiumpresent in the support, the impregnation temperature being comprisedbetween about 80° and 100° C.

The catalysts obtained according to these various processesadvantageously occur in the form of a powder consisting of particleswith a particle size similar to that of the support employed, and inparticular a particle size more or less identical to it.

According to the invention the prepolymer is obtained by contacting oneor more alpha-olefins with the catalyst and the co-catalyst consistingof organo-metallic compounds of a metal of Groups I to III of thePeriodic Table of Elements, such as organo-aluminium compounds. Theprepolymer contains, per gramme, between 2×10⁻³ and 10⁻¹gramme-milliatom of transition metal and preferably between 4×10⁻³ and3×10⁻² gramme-milliatom of transition metal.

The prepolymer must also occur in the form of a powder consisting ofparticles with a mean diameter by mass, Dm, comprised between 80 and 300microns for example between 100 and 300 microns, and preferablycomprised between 100 and 240 microns.

According to the invention the prepolymer must also occur in the form ofa powder consisting of particles having a particle size distributionsuch that the ratio Dm:Dn of the mean diameter by mass Dm to the meandiameter by number Dn is less than or equal to 3 and preferablycomprised between 1.1 and 2.5. Depending on the polymerization orcopolymerization conditions in the gas phase using a fluidised bed, itmay be preferable to use a prepolymer powder having a very narrowparticle size distribution, such that the ratio Dm:Dn is comprisedbetween 1.1 and 1.5, or else a prepolymer powder having a less narrowparticle size distribution such that the ratio Dm:Dn is comprisedbetween 1.5 and 2.5. In one embodiment of the present invention it ispreferred to use prepolymer having a particle size distribution suchthat the ratio Dm/Dn is less than or equal to 1.3. Preferably theprepolymer powder contains practically no particles with a diametergreater than 2×Dm and less than 0.2×Dm. The particle size distributionof the prepolymer powder may also be such that more than 90% by weightof the particles of one and the same batch are comprised in the bracketDm±10%.

The prepolymers described according to the invention have the advantageof not containing mineral compounds based on refractory oxides, such asalumina silica, aluminium silicate or magnesia oxide.

The prepolymers are obtained when one or more alpha-olefins are broughtinto contact with the catalyst and co-catalyst. This operation known asprepolymerization may be carried either in suspension in a liquidmedium, such as aliphatic hydrocarbons, or liquid alpha-olefins, or inthe gas phase.

As co-catalyst organo-aluminium compounds may be used, such as those ofthe formula Al(R₁₄)₃, in which R₁₄ is an alkyl radical having 2 to 12carbon atoms. Preferably organo-aluminium compounds are employed whichare not readily volatile, such as for example tri-n-octylaluminium.

In the particular case of production of polypropylene or propylenecopolymer, the co-catalyst employed is preferably an organo-aluminiumcompound complexed by an electron donor compound of the aromatics acidester type. The molar ratio between the electron donor compound and theorgano-aluminium compound is comprised between 0.1 and 0.5 andpreferably equal to about 0.3. An inadequate quantity of electron donorcompound decreases the stereospecificity of the catalyst system and toogreat a quantity, on the other hand, diminishes the activity of thecatalyst system.

In this prepolymerization operation the relative molar quantities oforgano-aluminium compound in relation to the transition metal compoundemployed may vary within very wide fields; for example the atomic ratioAl:transition metal may vary between 0.5 and 200.

Prepolymerization can be carried out to advantage in two stages.

The first stage of prepolymerization, or catalyst coating stage, ispreferably carried out under conditions such that the reaction speedsare relatively slow. The part played by this stage, whilst scrupulouslypreserving the shape of the catalyst particles, but on a larger scale,is to produce a catalyst known as a coated catalyst, having betterproperties for the subsequent stages of fluidised-bed polymerization,these properties being in particular an adequate mechanical strength, asuitable resistance to abrasion, an apparent density compatible with thefluidisation conditions and a controlled activity.

The coating stage, if any, is perforce effected by the polymerization orcopolymerization of alpha-olefins in suspension in a liquid medium. Thisstage, generally speaking, may continue until the coated catalystobtained contains from 0.1 to 10 g of polymer or copolymer pergramme-milliatom of transition metal.

The second stage of prepolymerization may take place either insuspension in a liquid medium or in the gas phase; generally this stagemay be continued, whilst preserving a suitable activity in the catalyst,until the prepolymer contains, per gramme, between 2×10⁻³ and 10⁻¹preferably between 4×10⁻³ and 3×10⁻² gramme-milliatom of transitionmetal.

Various means in themselves known may be employed in order to obtainprepolymer powders having, in particular, a particle size distributionas defined according to the invention. Among other things one may useprocesses involving selecting the desired particle size, such asscreening, or granulometric fractionation by means of a gas stream or bymeans of a stream of liquid. These particle size selection operationsmay be performed either on the prepolymer or on the catalyst, or whereappropriate on the support from which it originated. They are preferablycarried out under conditions such that the catalyst systems present inthese prepolymer powders do not lose their activity during theseoperations. In particular, the gases or liquids involved must becompletely inert in respect of these catalyst systems.

However, it is preferable for the sake of greater simplicity and betterprocess efficacity as a whole, to employ for the prepolymerization ofthe catalysts solids having a particle size distribution such that theydirectly produce the desired prepolymer powder. It is in fact importantduring the prepolymerization operations to control the process so thatthe prepolymer particles develop in a regular way to the extent thatthey have similar shapes to the original catalyst particles, but on alarger scale. The result is that the prepolymer obtained has a suitableparticle size distribution similar to that of the catalyst from which itoriginated, and that it is therefore directly ready for use.

The prepolymer as described according to the invention is then broughtinto contact with one or more alpha-olefins in the polymerization orcopolymerization stage in the gas phase, by means of a fluidised bed.This operation is advantageously performed continuously by techniques inthemselves known, according to which the gaseous mixture containing thealpha-olefins to be polymerized circulates with a gas stream risingthrough a fluidised bed consisting of particles of polymer or copolymerbeing formed. The alpha-olefins to be polymerized are introduced intothe fluidised-bed reactor at a temperature such that the reaction mediumis at a temperature of at least 60° C. and advantageously at least 80°C.

The speed of fluidisation in the fluidised bed reactor is preferablyhigh enough to ensure homogenisation of the fluidised bed and to be ableto eliminate effectively the heat given off by the polymerizationwithout having recourse to any other homogenisation method, particularlymechanical. The speed of fluidisation is preferably equal to 5 to 10times the minimum speed of fluidisation, that is to say generallycomprised between about 40 and 80 cm/sec. In traversing the fluidisedbed, only a part of the alpha-olefins polymerizes in contact with theparticles of polymer or copolymer in process of growth. The fraction ofthe alpha-olefins which has not polymerized leaves the fluidised bed andpasses through a cooling system intended to eliminate the heat producedduring the reaction, prior to being recycled into the fluidised bedreactor by means of a compressor.

The mean pressure in the reactor may be close to atmospheric pressure,but is preferably higher in order to increase the speed ofpolymerization. For example it may attain 3 MPa.

According to the invention the polymerization or copolymerization isadvantageously stopped when the polymer or copolymer contains, pergramme, less than 5×10⁻⁴ and preferably less than 2×10⁻⁴gramme-milliatom of transition metal.

The polymer or copolymer thus obtained occurs in the form of a powderconsisting of particles having a mean diameter by mass Dm comprisedbetween 300 and 1500 microns and preferably comprised between 600 and1200 microns and having a particle size distribution such that the ratioDm:Dn is less than or equal to 3.5 and preferably is comprised between1.2 and 3. The width of particle size distribution of the powder makingup the fluidised bed depends not only on that of the prepolymer employedbut also on the mean residence time of the polymer or copolymer in thefluidised-bed reactor, and also on the speed at which the catalystsystem loses its activity during the polymerization or copolymerizationreaction. In particular it is advantageous in such a process to use acatalyst system which loses its activity comparatively rapidly duringthe reaction in order amongst other things, to obtain a powder with thenarrowest possible particle size.

With a view to obtaining control of the molecular weight of the polymersor copolymers it is possible during the preparation of the prepolymerand in the fluidised-bed polymerization or copolymerization stage to mixthe alpha-olefins to be polymerized or copolymerized with achain-transfer agent such as hydrogen, in a molar ratio ofhydrogen:alpha-olefins comprised for example between 10 and 80%.

In some cases, particularly when the quantity of prepolymer used in thepolymerization is small, it may be convenient to mix a prepolymer withpolymer or copolymer which has already been formed, coming from aprevious operation, in order to be able to commence polymerization inthe fluidised bed with a quantity of solid material corresponding to anadequate height of the fluidised bed.

According to the process of the invention it is possible to produceunder satisfactory industrial conditions which are very simplified, alarge number of polymers and copolymers of alpha-olefins of a veryreproducible quality, and in particular:

high-density polyethylenes (density greater than 0.940), among which onemay identify homopolymers of ethylene and copolymers of ethylene andalpha-olefins having from 3 to 8 carbon atoms;

linear low-density polyethylenes (density less than 0.940), consistingof copolymers of ethylene and one or more alpha-olefins having 3 to 8carbon atoms, the molar content of units derived from ethylene beingequal to or greater than 90%;

elastomeric terpolymers of ethylene, propylene and dienes;

elastomeric copolymers of ethylene and propylene, having a content byweight of units derived from ethylene comprised between about 30 and70%;

isotactic polypropylenes and copolymers of propylene and ethylene orother alpha-olefins, having a content by weight of units derived frompropylene equal to or greater than 90%;

copolymers of propylene and butene-1 having a content by weight of unitsderived from butene-1 comprised between 10 and 40%.

Generally, ethylene homopolymers and ethylene/alpha-olefin copolymershaving a major proportion of ethylene units prepared by the process ofthe present invention are powders containing less than 350 ppm,preferably less than 150 ppm of inorganic residues which are free fromrefractory oxide support materials. The polyolefin powders have a highbulk density which can be, for example, at least 0.40 grammes per cubiccm and is frequently at least 0.45 grammes per cubic centimeter.

Method of determining the mean diameters by mass (Dm) and by number (Dn)of the particles (support, catalyst, prepolymer or polymer)

According to the invention the mean diameters by mass (Dm) and by number(Dn) of the particles of support, catalyst, prepolymer or polymer aremeasured on the basis of microscope examinations, by means of theOPTOMAX image analyser (Micro-Measurements Ltd, Great Britain). Themeasuring principle consists in obtaining, from the experimental studyby optical microscope, of a population of particles, a table offrequencies giving the number (ni) of particles belonging to each class(i) of diameter, each class (i) being characterised by an intermediatediameter (di) comprised between the limits of the said class.

According to the approved French Standard NF X 11-630 of June 1981, Dmand Dn are provided by the following formulae:

mean diameter by mass: ##EQU1## mean diameter by number: ##EQU2##

The ratio Dm:Dn characterises the particle size distribution; it issometimes referred to as the "width of particle size distribution."

Measurement by the OPTOMAX image analyser is carried out by means of aninverted microscope which makes it possible to examine suspensions ofparticles of support, catalyst, prepolymer or polymer at an enlargementcomprised between 16 and 200×. A television camera picks up the imagesgiven by the inverted microscope and transmits them to a computer whichanalyses the images received line by line and dot by dot on each line inorder to determine the dimensions of diameters of the particles and thento classify them.

The following non-restrictive examples illustrate the invention.

EXAMPLE 1 Preparation of the Support

Into a 5-liter stainless steel reactor equipped with an agitation systemturning at 750 revolutions per minute and containing 800 ml of n-hexane,there is introduced, at ambient temperature (20° C.) and under a blanketof nitrogen, 1725 ml of a solution of butyloctylmagnesium in n-hexanecontaining 1500 gramme milliatoms of magnesium, and 153 ml (750millimoles) of di-isoamyl ether. The reactor is then heated to 50° C.and over 3 hours 322 ml of t-butyl chloride (or 2925 milliatoms) isprogressively added.

At the end of this addition, the suspension is maintained at 50° C. for3 hours and the precipitate obtained is washed with n-hexane.

The solid product (A) obtained has the following composition per grammeatom of magnesium: 1.97 gramme atoms of chlorine, 0.03 grammeequivalents of Mg-C bonds and 0.02 mole of di-isoamyl ether.

On microscopic examination, the solid product (A) was seen to be in theform of a powder consisting of spheroidal particles (the mean ratiobetween the large and small axes, D/d, of the particles is equal to1.2), having a narrow particle size distribution, defined by the ratioDm/Dn=1.1 with Dm=52 microns; it is found furthermore that more than 90%by weight of the particles have a mean diameter comprised between 47 and57 microns; the density of the product is equal to 1.9 and its specificsurface area to 38 m^(2/g) (BET); the surface of the particles isperfectly smooth.

EXAMPLE 2

As catalyst support there was used the product (A) prepared as inExample 1 above.

Production of the catalyst

To 3000 ml of suspension in n-hexane of product (A), containing 1450milliatoms of MgCl₂, there is added with agitation 82 ml of di-isoamylether and 400 ml of a 1.2 molar solution of diethylaluminium chloride inn-hexane (or 480 milliatoms). The reactor is brought to 50° C., andthere is introduced progressively over 2 hours, 650 ml of a 0.6 molarsolution of di-n-propoxytitanium dichloride in n-hexane (or 390millimoles). At the end of this addition the temperature is brought to80° C. and the mixture is maintained at this temperature for 2 hours.The catalyst obtained is then washed five times with n-hexane to givethe catalyst solid ready for use (B). Analysis of the catalyst (B) showsthat it contains per gramme atom of total titanium: 0.94 gramme atom oftrivalent titanium; 0.06 gramme atom of tetravalent titanium; 3.85gramme atoms of magnesium; 9.97 gramme atoms of chlorine; 0.20 grammeatom of aluminium; and 0.11 mole of di-isoamyl ether. The catalyst thusdefined is a brown powder consisting of grains of spheroidal shape,having a narrow particle size distribution such that more than 90% ofthe particles have a mean diameter comprised between 50 and 60 microns,with Dm=55 microns; it is found, furthermore, that the ratio Dm/Dn ofthe catalyst particles is equal to 1.2; the surface of the particles isperfectly smooth.

Prepolymerization (first stage)

Into a 5-liter stainless steel reactor equipped with an agitation systemturning at 750 revolutions per minute and containing 2 liters ofn-hexane heated to 50° C., there is introduced under a blanket ofnitrogen 100 millimoles of tri-n-octylaluminium (TnOA) and a suspensionof catalyst (B) in hexane containing 500 gramme-milliatoms of titanium.The reactor is heated to 60° C. and ethylene is introduced at a constantrate equal to 167 g per hour, over 3 hours. At the end of the reactionthe whole is decanted into a rotary evaporator under vacuum; in this way820 g of dry powder (C) of a light brown coloured prepolymer areobtained, consisting of particles with a mean diameter by mass equal to66 microns and a narrow particle size distribution, such that the ratioDm/Dn is equal 1.2. The powder (C) is preserved under nitrogen.

Prepolymerization (second stage)

Into a fluidised bed reactor with a diameter of 15 cm operating with agas speed of 10 cm/sec such that the partial pressures of the componentsare 0.8 MPa nitrogen, 0.1 MPa of hydrogen and 0.1 MPa of ethylene, every6 minutes there is introduced 11g of the powder (C) and continuously 25g per hour of pure TnOA, into the bottom half of the bed maintained at70° C. During intermittent withdrawals there are collected 4 kg per hourof a slightly beige-tinted powder which shows, for a residence time ofhalf an hour in the reactor, a titanium content of 800 ppm of titanium(or 1.67×10⁻² gramme-milliatoms of titanium per gramme of prepolymer), amean diameter by mass of 260 microns, a particle size distribution suchthat the ratio Dm/Dn is equal to 1.3 and a bulk density of 0.41 g/cm³ ;the pre-polymer (D) thus obtained is also preserved under nitrogen.

Polymerization of ethylene

Into a fluidised bed reactor with a diameter of 46 cm operating with arising gas propelled at a speed of 45 cm/sec and under partial pressuresof 1.2 MPa of hydrogen and 0.8 MPa of ethylene, there is introduced in asequential manner 0.5 kg per hour of prepolymer (D) into the bedmaintained at 85° C. During the intermittent withdrawals there arecollected 25 kg per hour of a white powder which, for a residence timeof 6 hours in the reactor, shows a content of 16 ppm of titanium (or3.3×10⁻⁴ gramme-milliatoms of titanium per gramme of polymer), a meandiameter by mass of 940 microns, a narrow particle size distributionsuch that the ratio Dm/Dn is equal to 1.5 and a bulk density of 0.47g/cm³ ; moreover the melt index under 2.16 kg at 190° C. is 6 g/10minutes and the molecular weight distribution, Mw/Mn, measured by GPC,is equal to 4.0.

The produced polymers contained about 218 ppm of inorganic residue.

EXAMPLE 3

There is used as support (A) a powder based on magnesium chloride,consisting of spherical particles having a narrow particle sizedistribution, such that more than 90% by weight of the particles have amean diameter comprised between 29 and 35 microns, with Dm=32 microns;moreover, it is ascertained that the ratio Dm/Dn is equal to 1.1; thispowder shows a density equal to 1.85 and a specific surface area equalto 41 m² /g (BET).

Preparation of the catalyst

This is identical to that of Example 2. Analysis of the catalyst (B)obtained gives per gramme-atom of total titanium: 0.96 gramme-atom oftrivalent titanium; 0.04 gramme-atom of tetravalent titanium; 3.60gramme-atom of magnesium; 9.40 gramme-atoms of chlorine; 0.13gramme-atom of aluminium and 0.07 mole of di-isoamyl ether. The catalyst(B) is a brown-coloured powder composed of spherical particles, having anarrow particle size distribution such that more than 90% by weight ofthe particles have a mean diameter comprised between 30 and 36 micronswith Dm=33 microns; it is ascertained furthermore that the ratio Dm/Dnis equal to 1.2; the surface of the catalyst particles is slightlydented, of the "raspberry" type.

Prepolymerization (first stage)

This is identical to that of Example 2. 807 g of a dry powder (C) of aprepolymer, having a mean diameter by mass equal to 40 microns and anarrow particle size distribution, such that the ratio Dm/Dn is equal to1.2, are obtained.

Pre-co-polymerization (second stage)

Into a fluidised bed reactor with a diameter of 15 cm operating with arising gas propelled at a speed of 10 cm/sec and under partial pressuresof 1 MPa of nitrogen, 0.04 MPa of hydrogen, 0.05 MPa of butene-1 and0.13 MPa of ethylene, there are introduced every 6 minutes 6.5 g of thepowder (C) and continuously 26.4 g per hour of TnOA into the bedmaintained at 70° C. There was withdrawn 4 kg per hour of powder (D) ofa pre-copolymer of ethylene which contained, per gramme, 10⁻²gramme-milliatom of titanium, for a residence time of half an hour inthe reactor. This powder had a mean diameter by mass of 190 microns, aparticle size distribution such that the ratio Dm/Dn is equal to 1.3 anda bulk density of 0.36 g/cm³.

Copolymerization of ethylene and butene-1

Into a fluidised bed reactor with a diameter of 46 cm operating with arising gas propelled at a speed of 45 cm/sec and under partial pressuresof 0.7 MPa of nitrogen, 0.2 MPa of hydrogen, 0.26 MPa of butene-1 and0.84 MPa of ethylene, there are introduced 0.44 kg/hr of thepre-copolymer (D) into the bed maintained at 80° C. On withdrawal thereare collected 21 kg per hour of a powder of copolymer of ethylene whichcontains, per gramme of copolymer, 2.1×10⁻⁴ gramme-milliatoms oftitanium, for a residence time of 6 hours in the reactor. This copolymerpowder has a mean diameter by mass of 720 microns and a particle sizedistribution such that the ratio Dm/Dn of the particles is equal to 1.6;moreover, melt index under 2.16 kg at 190° C. is equal to 1 g/10 mins;its density at 20° C. is equal to 0.917 and its bending strength is 21MPa; the bulk density is equal to 0.40 g/cm³. The produced copolymercontained 136 ppm of inorganic residue.

EXAMPLE 4

There is used as support (A) a powder based on magnesium chloride,consisting of spherical particles having a particle size distributionsuch that the ratio Dm/Dn is equal to 2.3 with Dm=23 microns; thispowder has a density equal to 2.1.

Preparation of the Catalyst

This is identical to that of Example 2. Analysis of the product (B)obtained gives per gramme-atom of total titanium: 0.94 gramme-atom oftrivalent titanium; 0.06 gramme-atom of tetravalent titanium, 3.80gramme-atoms of magnesium, 9.84 gramme atoms of chlorine, 0.16gramme-atom of aluminium and 0.08 mole of di-isoamyl ether. The catalyst(B) is a brown powder composed of spheroidal particles, having aparticle size distribution such that the ratio Dm/Dn is equal to 2.4,with Dm=23 microns.

Prepolymerization (first stage)

This is identical to that of Example 2. There are obtained 817 g of drypowder (C) of a prepolymer having a mean diameter by mass equal to 28microns and a particle size distribution such that the ratio Dm/Dn isequal to 2.4.

Prepolymerization (second stage)

Into a fluidised bed reactor with a diameter of 15 cm operating with arising gas propelled at a speed of 10 cm/sec., and under partialpressures of 1.8 MPa of nitrogen, 0.1 MPa of hydrogen and 0.1 MPa ofethylene, there are introduced every 5 minutes 2.2 g of powder (C) andcontinuously 28 g per hour of TnOA into the bottom half of the bedmaintained at 70° C. The powder (D) of prepolymer recovered onwithdrawal at the rate of 4 kg/h contains, per gramme, 4×10⁻³gramme-milliatoms of titanium, for a residence time of half an hour inthe reactor. This powder has a mean diameter by mass of 175 microns, aparticle size distribution such that the ratio Dm/Dn is equal to 2.5,and a bulk density of 0.42 g/cm³.

Polymerization of ethylene

This is identical to that of Example 2. There are obtained in this way apolyethylene containing, per gramme, 8×10⁻⁵ gramme-milliatoms oftitanium; the polyethylene powder has a mean diameter by mass of 640microns, a particle size distribution such that the ratio Dm/Dn of theparticles is equal to 2.8 and a bulk density of 0.51 g/cm³ ; moreoverthe melt index under 2.16 kg at 190° C. is 8 g/10 minutes. The producedpolyethylene contained about 54 ppm of inorganic residue.

Example 5

There is used as support (A) a powder based on magnesium chloride,consisting of spheroidal particles having a particle size distributionsuch that the ratio Dm/Dn is equal to 1.3, with Dm=23 microns.

Preparation of the catalyst

Into 3000 ml of suspension in n-hexane of product (A) containing 1450millimoles of Mg Cl₂, there is added under agitation 82 ml of di-isoamylether and 330 ml of a 1.2 molar solution of diethyl aluminium chloridein n-hexane (or 396 millimoles). The reactor is brought to 50° C. andthere is introduced progressively over 2 hours 650 ml of a 0.6 molarsolution of di-n-propoxytitanium dichloride in n-hexane (or 390millimoles). At the end of this addition, the temperature is brought to80° C. and maintained there for 2 hours. The catalyst obtained is thenwashed five times with n-hexane to give the catalytic solid ready foruse (B). Analysis of the catalyst (B) obtained shows that it containsper gramme-atom of total titanium: 0.94 gramme-atom of trivalenttitanium, 0.06 gramme-atom of tetravalent titanium, 3.80 gramme-atoms ofmagnesium, 9.90 gramme-atoms of chlorine, 0.20 gramme-atom of aluminiumand 0.10 mole of di-isoamyl ether. The catalyst thus defined is abrown-coloured powder composed of particles of spheroidal shape, havinga particle size distribution such that the ratio Dm/Dn is equal to 1.3,with Dm=23 microns; the surface of the particles is smooth.

Prepolymerization (first stage)

Into a 5-liter stainless steel reactor, fitted with an agitation systemturning at 750 revolutions per minute and containing 2 liters ofn-hexane heated to 50° C., there are introduced under a blanket ofnitrogen 80 millimoles of tri-n-octyl aluminium (TnOA) and a suspensionof catalyst (B) in hexane containing 80 gramme-atoms of titanium [or 46g of (B)]. The reactor is heated to 60° C. and ethylene is introduced ata constant rate equal to 167 g/hour for a period of 3 hours. At the endof the reaction the whole is decanted into a rotary evaporator undervacuum, in this way are obtained 570 g of dry powder (C) of a lightbrown-coloured prepolymer, composed of particles of a mean diameter bymass equal to 50 microns and an particle size distribution such that theratio Dm/Dn is equal to 1.4. The powder (C) is preserved under nitrogen.

Prepolymerization (second stage)

Into a fluidised bed reactor with a diameter of 15 cm operating with agas speed of 10 cm/sec, under partial pressures of 0.8 MPa of nitrogen,0.1 MPa of hydrogen and 0.1 MPa of ethylene, there are introduced everysix minutes 12 g of the powder (C) into the bottom half of the bedmaintained at 70° C. In a series of withdrawals, there are collected 40kg/hour of a slightly beige-tinted powder (D) which contains, pergramme, 4×10⁻³ gramme-milliatoms of titanium, for a residence time of 2hours in the reactor. This powder has a particle size distribution suchthat the ratio Dm/Dn of the particles is equal to 1.6, with Dm=172microns, and a bulk density of 0.41 g/cm³ ; the prepolymer (D) thusobtained is also preserved under nitrogen.

Polymerization of ethylene

Into a fluidised bed reactor with a diameter of 46 cm operating with arising gas propelled at a speed of 45 cm/sec and under partial pressuresof 1.2 MPa of hydrogen and 0.8 MPa of ethylene, there are introduced ina sequential manner 0.5 kg/hour of prepolymer (D) and continuously 29.8g/hour of TnOA into the bottom half of the bed maintained at 85° C. Onsequential withdrawal, there are recovered 25 kg/hour of white powderwhich contains, per gramme, 8×10⁻⁵ gramme-milliatoms of titanium, for aresidence time of 5 hours in the reactor. This powder has a particlesize distribution such that the ratio Dm/Dn of the particles is equal to1.8, with Dm=650 microns, and a bulk density of 0.52 g/cm³ ; moreover,the melt index under 2.16 kg at 190° C. is 6 g/10 minutes and themolecular weight distribution, Mw/Mn, measured by GPC, is equal to 4.

The produced polymer contained about 54 ppm of inorganic residue.

Example 6

The support (A) is used prepared as in Example 5.

Preparation of catalyst

Into 3000 ml of suspension of support (A) containing 1450 millimoles ofMgCl₂, there are added under agitation 8.2 ml of di-isoamyl ether and270 ml of a 1.2 molar solution in n-hexane of diethylaluminium chloride.The reaction medium is heated to 30° C. and over 2 hours 650 ml of a 0.6molar solution of titanium tetrachloride are added progressively. At theend of this addition, the medium is heated to 80° C. and maintainedunder agitation at this temperature for 2 hours. The catalyst obtainedis then washed five times with n-hexane to produce the catalyst solid(B) ready for use.

Analysis of the catalyst (B) shows that it contains per gramme-atom oftotal titanium: 0.97 gramme-atom of trivalent titanium, 0.03 gramme-atomof tetravalent titanium, 3.7 gramme-atoms of magnesium, 11.2 grammeatoms of chlorine, 0.33 gramme-atom of aluminium, and 0.01 mole ofdi-isoamyl ether.

The catalyst thus defined is a brown-coloured powder composed ofspheroidal particles, having a particle size distribution such that theratio Dm/Dn is equal to 1.4, with Dm=23 microns.

Prepolymerization in suspension

Into a 5-liter stainless steel reactor there are introduced 3000 ml ofn-hexane which is heated to 70° C. with agitation (750 revolutions perminute), 19 ml of a 1.14 molar solution in n-hexane oftri-n-octylaluminium, 16.7 ml of a suspension of the catalyst (B)containing 0.13 gramme-atom of titanium per liter and a volume of 1500ml of hydrogen, measured under normal conditions. Ethylene is thenintroduced at a throughput of 180 g/hr for 3 hours, together with 1500ml of hydrogen, measured under normal conditions, after 11/2 hours ofreaction. The prepolymer obtained is dried at 70° C. under nitrogen, thequantity collected being 530 g. The prepolymer powder contains, pergramme, 4.1×10⁻³ gramme-milliatoms of titanium. It has a particle sizedistribution such that the ratio Dm/Dn of the particles is equal to 1.8,with Dm=180 microns. The bulk density is equal to 0.36 g/cm³.

Polymerization of ethylene

This is identical to that of Example 4 except that the prepolymerprepared in Example 6 is used, and that the pressures for hydrogen andethylene are 0.6 and 1.4 MPa respectively. The prepolymer is introducedinto the fluidised bed at a throughput of 0.470 kg/hr and the productionis 30 kg/hr of polyethylene. The polymer obtained has a mean diameter bymass of 700 microns, a bulk density of 0.44 g/cm³, a melt index under 5kg at 190° C. of lg/10 mins and a molecular weight distribution, Mw/Mn,equal to 10. The polymer contains, per gramme, 6×10⁻⁵ gramme-milliatomsof titanium.

The polymer was found to contain about 40 ppm of inorganic residue.

Example 7 Preparation of the support

Into a glass reactor, having a capacity of 1 liter, fitted with anagitation system turning at 500 revolutions per minute, there isintroduced at ambient temperature and under nitrogen 550 ml of asolution in n-hexane of dibutyl magnesium containing 500gramme-milliatoms of magnesium and 51 ml of di-isoamyl ether (250millimoles).

The reactor is then heated to 50° C. and there is introduced, drop bydrop over 2 hours, 115 ml of tertiary butyl chloride (1050 millimoles).At the end of this addition, the suspension is maintained at 50° C. for2 hours and the precipitate obtained is washed at the same temperaturewith n-hexane.

The support thus formed contains per gramme-atom of magnesium: 2.0gramme-atoms of chlorine and 0.011 mole of di-isoamyl ether.

Under microscopic examination, the support is seen to be in the form ofspheroidal particles (the ratio D/d between the large and small axes ofthe particles is on average equal to 1.2) having a particle sizedistribution such that Dm/Dn equals 1.2 with Dm=60 microns; it is foundthat more than 90% by weight of the particles have a mean diametercomprised between 54 and 66 microns; these particles have a smoothsurface, a specific surface area equal to 42 m² /g (BET) and a densityequal to 1.3.

Preparation of the catalyst

Into a glass reactor, having a capacity of 1 liter and fitted with anagitation system turning at 250 revolutions per minute, there isintroduced under nitrogen 500 ml of a suspension in n-hexane of thesupport prepared as described above, this suspension containing 0.2gramme-atoms of magnesium. After decantation, the supernatanthydrocarbon phase is withdrawn. The reactor is then heated to 50° C. andthere are introduced 2 ml of ethyl benzoate (14 millimoles). Thesuspension is maintained under agitation for 2 hours, then there areintroduced 2 moles of pure titanium tetrachloride (220 ml). Thetemperature is raised to 80° C., and this temperature is maintained for2 hours. The solid obtained is then washed with n-hexane at 50° C. togive the catalyst ready for use, in the form of a suspension inn-hexane.

The analysis of the catalyst shows that it contains, per gramme atom ofmagnesium: 2.05 gramme atoms of chlorine, 0.014 gramme-atoms oftitanium, 0.016 mole of ethyl benzoate and that it contains no trace ofdi-isoamyl ether.

The catalyst thus defined is a yellow-greyish coloured powder,consisting of particles of spheroidal shape, having a particle sizedistribution such that Dm/Dn equals 1.2 with Dm=60 microns; it is foundmoreover that more than 90% by weight of the particles have a meandiameter comprised between Dm±10%; these particles have a surface assmooth as that of the initial support.

Prepolymerization in suspension

Into a stainless steel reactor, having a capacity of 5 liters and fittedwith an agitation system turning at 750 revolutions per minute, thereare introduced under cover of nitrogen 25 millimoles of tri-isobutylaluminium (TiBA), 9.25 millimoles of methyl paratoluate and a quantityof the catalyst prepared as in Example 7 corrsponding to 2.5gramme-milliatoms of titanium. The suspension is made up to 2 literswith n-hexane. There is introduced into the reactor at ambienttemperature (20° C.) a volume of 30 ml of hydrogen measured under normalconditions, then propylene at a rate of 200 g/hour for 21/2 hours. Atthe end of this time, the suspension of prepolymer is maintained underagitation for a further half hour. The reactor is de-gassed and, alwaysunder cover of nitrogen, the prepolymer powder is washed three timeswith n-hexane. The suspension of prepolymer in n-hexane is then decantedinto a rotary evaporator under vacuum. There are obtained 510 g of a dryprepolymer powder, consisting of spheroidal particles, of a particlesize distribution such that the ratio Dm/Dn is equal to 1.4, of a meandiameter by mass equal to 175 microns, having a smooth surface andcontaining, per gramme of prepolymer, 5×10⁻³ gramme-milliatoms oftitanium. This powder is preserved under nitrogen.

Polymerization of propylene

Into a fluidised bed reactor, of a diameter of 46 cm, operating with arising gas propelled at a speed of 45 cm/sec and under partial pressuresof 0.1 MPa of hydrogen and 1.5 MPa of propylene, there are introduced ina sequential manner 0.8 kg/hour of the dry prepolymer powder prepared asin Example 7. There is introduced continuously a solution in n-hexane ofa mixture of TnOA and of methyl paratoluate in a molar ratio 1/0.25, ata rate corresponding to 450 millimoles of TnOA per hour. The temperatureof the fluidised bed is maintained at 60° C. during the wholepolymerization. On intermittent sequential withdrawal, there areobtained about 25 kg/hour of a dry polypropylene powder directly useablewhich contains, per gramme, 1.6×10⁻⁴ gramme-milliatoms of titanium, fora residence time of 6 hours in the reactor. This polypropylene powderhas a mean diameter by mass equal to 400 microns, a bulk density of 0.45g/cm³, a residue insoluble in boiling n-heptane of 90% by weight and amelt index under 5 kg at 190° C. of 2 g/10 minutes.

EXAMPLE 8 Copolymerization of propylene and ethylene

One operates exactly as in Example 7, and in particular with the sameprepolymer, except that the fluidised bed reactor operates under partialpressures of 1.4 MPa of propylene and 0.1 MPa of ethylene instead of 1.5MPa of propylene alone, and that there are introduced into the fluidisedbed reactor 0.6 kg/hour of prepolymer instead of 0.8 kg/hour.

On intermittent withdrawal, there are obtained about 20 kg/hour of a drypowder of a copolymer of propylene and ethylene directly useable whichcontains, per gramme, 1.5×10⁻⁴ gramme-milliatoms of titanium. Thiscopolymer powder has a mean diameter by mass of 410 microns, a bulkdensity of 0.44 g/cm³, a content of units derived from ethylene of 5% byweight, a residue of copolymer in soluble in boiling n-heptane of 85% byweight and a melt index under 5 kg at 190° C. of 3 g/10 minutes.

We claim:
 1. Process for the manufacture of polyolefins directly in theform of a powder consisting of particles having a mean diameter by masscomprised between 300 and 1,500 microns, a content of inorganic residuesof less than 350 ppm and a content per gramme of less than 5×10⁻⁴gramme-milliatoms of transition metal and a particle size distributionsuch that the ratio of the mean diameter by mass, Dm, to the meandiameter by number, Dn, is less than or equal to 3.5 comprisingin aprepolymerization contacting one or more alpha-olefins with a Zieglercatalyst system comprising a solid catalyst in the form of particlescontaining essentially atoms of halogen, magnesium and a transitionmetal of Groups IV, V and VI of the Periodic Table of Elements, and, acocatalyst consisting of an organometallic compound of a metal of GroupsI and III of the Periodic Table, to obtain a prepolymer in a form ofparticles, wherein said prepolymerization is carried out in two stages,said first stage being a catalyst coating stage which is effected by thepolymerization or copolymerization of alpha-olefins in suspension in aliquid medium, with this stage continuing until the coated catalystobtained contains from 0.1 to 10 g of polymer or copolymer pergramme-milliatom of transition metal, said second stage ofprepolymerization taking place in the gas phase, and this stagecontinuing while preserving a suitable activity in the catalyst, untilthe prepolymer contains, per gramme, between 2×10⁻³ and 10⁻¹gramme-milliatom of transition metal, then, in a final polymerizationcontacting the said prepolymer with one or more alpha-olefins undercontinuous polymerization or copolymerization conditions in the gasphase by means of a fluidized bed, to produce continuously thealpha-olefin polymer or copolymer particles, characterized in that: (a)the prepolymer contains, per gramme, between 2×10⁻³ and 10⁻¹gramme-milliatoms of transition metal and is in the form of particleshaving a mean diameter by mass comprised between 80 and 300 microns anda particle size distribution such that the ratio of the mean diameter bymass, Dm, to the mean diameter by number, Dn, is less than or equal to3, and (b) the particles of prepolymer and polymer or copolymer aremaintained in the fluidized state solely by means of an ascending gasflow containing the alpha-olefin or alpha-olefins to be polymerized andhaving a velocity between 40 and 80 cm/sec.
 2. Process according toclaim 1 characterised in that the prepolymer is in the form of particleshaving a mean diameter by mass comprised between 100 and 240 microns. 3.Process according to claim 1 characterised in that the prepolymer has aparticle size distribution such that the ratio Dm/Dn of the meandiameter by mass to the mean diameter by number of the particles iscomprised between 1.1 and 2.5.
 4. Process according to claim 1characterised in that the prepolymer has a particle size distributionsuch that the ratio Dm/Dn is comprised between 1.1 and 2.5.
 5. Processaccording to claim 1 characterised in that the prepolymer has a particlesize distribution such that the ratio Dm/Dn is comprised between 1.1 and1.5.
 6. Process according to claim 1 characterised in that theprepolymer is in the form of particles having a particle sizedistribution such that more than 90% by weight of the particles have amean diameter comprised within the range Dm±10%.
 7. Process according toclaim 1 characterised in that the prepolymer contains no mineralcompounds derived fromrefractory oxides
 8. Process according to claim 1characterised in that the alpha-olefin polymer or copolymer has aparticle size distribution such that the ratio Dm/Dn is comprisedbetween 1.2 and
 3. 9. Process according to claim 1 characterised in thatthe catalyst comprises a solid support consisting essentially of amagnesium compound, on which has been deposited a compound of atransition metal from Groups IV, V and VI of the Periodic Table ofElements.
 10. Process according to claim 9 characterised in that themagnesium compound is magnesium chloride.
 11. Process according to claim9 characterised in that the transition metal compound is a titaniumcompound.
 12. Process in accordance with claim 11, characterised in thatthe titanium compound is deposited on the solid support byprecipitation, this precipitation being carried out in the presence ofthe solid support by a reaction comprising reducing maximum valencytitanium compound of the formula Ti (OR₇) (_(4-n))X_(n) in which R₇ isan alkyl group containing 2 to 6 carbon atoms, X is a chlorine orbromine atom and n is an integer or fraction from 1 to 4 inclusive, witha reducing agent chosen from among the organo-magnesium compounds of theformula R₈ MgR₉ in which R₈ and R₉ are alkyl groups with 2 to 12 carbonatoms, organo-zinc compounds of the formula Zn(R₁₀) (_(2-y))X_(y) inwhich R₁₀ is an alkyl group with 2 to 12 carbon atoms, X is chlorine orbromine and y is 0 or 1 or a fraction less than 1, and organo-aluminiumcompounds of the formula Al(R.sub. 11) (_(3-x))X_(x) in which R₁₁ is analkyl group with 2 to 12 carbon atoms, X is chlorine or bromine and x is0 or an integer or fraction not greater than 2, the said reaction beingperformed optionally in the presence of an electron donor compound,chosen from the organic compounds comprising at least one atom ofoxygen, sulphur, nitrogen and/or phosphorus
 13. Process in accordancewith claim 12, characterised in that the solid support is impregnatedwith an organo-magnesium, organo-zinc or organo-aluminium compound, andthe resulting product is then treated with the titanium compound of theformula Ti(OR₇) (_(4-n))X_(n).
 14. Process in accordance with claim 11,characterised in that the catalyst consists of a solid support based onmagnesium chloride which is, in a preliminary step, treated with anelectron donor compound of the aromatic acid ester or aromtic ethertype, and on which titanium tetrachloride has been deposited byimpregnation.
 15. Process in accordance with claim 11, wherein the solidsupport has a mean particle diameter by mass in the range 10-100microns, the prepolymer has a mean particle diameter by mass in therange 100-300 microns and the produced polymer or copolymer has a meanparticle diameter by mass in the range 300-1500 microns.
 16. Process inaccordance with claim 15 wherein the particle size distribution of theprepolymer is such that the ratio Dm/Dn is less than or equal to 1.3.17. Process in accordance with claim 15 wherein the particle sizedistribution of the support is such that more than 90% by weight of theparticles have a mean diameter comprised within the range Dm±10%.
 18. Aprocess according to claim 12, in which the electron donor compound isan aliphatic ether-oxide of the formula R₁₂ -O-R₁₃ in which R₁₂ and R₁₃which may be the same or different are chosen from alkyl groups with 1to 12 carbon atoms.
 19. The process according to claim 1, wherein saidalpha-olefin is selected from the group consisting of ethylene,propylene, a mixture of propylene and ethylene, and a mixture ofethylene and butene-1.